Imaging apparatus that are currently used create color images by separating incident light through the use of, for example, color filters of the primary colors of red, green, and blue. Color imaging apparatus are broadly divided into the following two types: so-called three-chip systems in which three identical image sensors are used as shown in FIG. 10, and so-called single-chip systems in which a single image sensor is used and a color filter is arranged for each pixel as shown in FIG. 11A. As image sensors, CCD (Charge Coupled Device) image sensors, CMOS (Complementary Metal Oxide Semiconductor) image sensors, and the like are used.
In the three-chip system shown in FIG. 10, incident light 11 that is the subject to be imaged enters a lens system 12. Among such light, light with red components is reflected by a red mirror 14a, and then enters a red, image sensor 13a to be imaged. Similarly, light with blue components is reflected, by a blue mirror 14b, and then enters a blue image sensor 13b to be imaged. Light with green components is not reflected by the red mirror 14a or the blue mirror 14b, but enters a green image sensor 13c to be imaged. As described above, light with red components, light with green components, and light with blue components enter the red, green, and blue image sensors, respectively, to be imaged. Combining such images will allow reproduction of the original subject to be imaged.
In the single-chip system shown in FIG. 11A, incident light 21 that is the subject to be imaged enters a lens system 22. Then, the light is transmitted and absorbed by a color filter 25 and enters an image sensor 23 to be imaged. The color filter 25 is composed of micro filters of red (R), green (G), and blue (B) as shown in FIG. 11B, which are arranged such that the position of each filter corresponds to the position of each pixel of the image sensor. The red filter transmits light with red components and absorbs light with other components. The green filter transmits light with green components and absorbs light with other components. The blue filter transmits light with blue components and absorbs light with other components. Consequently, light with red components, light with green components, and light with blue components enter their respective pixels to be imaged. Combining such images will allow reproduction of the original subject to be imaged. It should be noted that, the color filter shown in FIG. 11B is only exemplary. Typically, more green pixels than red and blue pixels are provided in order to provide high spatial frequency to luminance signals.
As in the aforementioned structure, the three-chip system, which requires three image sensors, has problems in that cost could increase and the size of the system could also increase due to the complex structure. The three-chip system, however, is capable of performing imaging with high light use efficiency compared to the single-chip system. Such points will be described with reference to FIGS. 12 and 13.
FIG. 12 are conceptual diagrams illustrating, the light use efficiency of the three-chip system. As shown in FIG. 12A, the horizontal axis represents the wavelength of incident light and the vertical axis represents the light intensity thereof. A state in which the light intensity is constant at various wavelengths is shown. When such light is reflected by the red mirror and enters the red image sensor, long-wavelength light with red components is obtained as shown in FIG. 12B. When loss by the mirror and the like is ignored, the light intensity of the obtained light can be considered as the same as that of the original incident light. Similarly, light reflected by the blue mirror becomes, upon entering the blue image sensor, short-wavelength light with blue components as shown in FIG. 12D. When loss by the mirror and the like is ignored, the light intensity of the obtained light can be considered that of the original incident light. Light with the rest green components, upon entering the green image sensor, becomes that shown in FIG. 12C. When loss by the mirror and the like is ignored, the light intensity of the obtained light can be considered as the same as that of the original incident light. Light that can be used to reproduce the subject to be imaged by electronically combining the light that has entered each image sensor is shown in FIG. 12E, which is the same as the incident light shown in FIG. 12A. Thus, quite high light use efficiency is ensured.
FIG. 13 are conceptual diagrams illustrating the light use efficiency of the single-chip system. As shown in FIG. 13A, the horizontal axis represents the wavelength of incident light and the vertical axis represents the light intensity thereof. The light is transmitted and absorbed by the filters. Light with red components is transmitted by the red filter, and it becomes, upon entering the image sensor, that shown in FIG. 13B. When loss by the filter and the like is ignored, the light intensity of the obtained light can be considered as the same as that of the original incident light. Similarly, light with green components is transmitted by the green filter, and it becomes, upon entering the image sensor, that shown in FIG. 13C. When loss by the filter and the like is ignored, the light intensity of the obtained light can be considered as the same as that of the original incident light. Light with blue components is transmitted by the blue filter, and it becomes, upon entering the image sensor, that shown in FIG. 13D. When loss by the filter and the like is ignored, the light intensity of the obtained light can be considered as the same as that of the original incident light.
However, if a filter with a pixel configuration (RGB) shown two-dimensionally in FIG. 11B, among which four pixels include one red pixel, two green pixels, and one blue pixel, is used for the single-chip system, for example, the intensity of light with red, green, and blue components is decreased to 1/4, 2/4, and 1/4, respectively as shown in FIGS. 13E, 13F, and 13G, in proportion to the areas of the pixels. Thus, light that can be used to reproduce the subject to be imaged by electronically combining the light that has entered each mage sensor is decreased to about 1/3 as shown in FIG. 13H, which greatly differs from the incident light shown in FIG. 13A. Thus, the light use efficiency is low.
Further, when color images are created using red, green, and blue color filters as described above, the resulting colors do not match the colors perceived by humans, and cannot cover the entire color range (color gamut) that can be perceived by humans. In order to solve such problems, a method as described in Patent Document 1 below is disclosed. In the patent document, XYZ color matching functions defined by the CIE are linearly transformed, and filters that are substantially equivalent to the XYZ color matching functions are used (see FIG. 14). Using such filters can realize a moving-image camera with a color gamut that is equal to the color gamut of humans.    Patent Document 1: JP Patent Publication (Kokai) No. 2005-260527 A